There's always been a heckuva lot of confusion regarding atheism, but lately, I've noticed people take it to who new levels. It's not uncommon among fundamentalist circles see atheism denounced as a religion. Even for your average person, misconceptions abound. For many, it seems they think that atheism is a "system of belief" or some miraculously universal worldview that all atheists share.
Christian apologists in particular seem to have a hard time with atheism. I don't think they can quite grasp the concept of being irreligious - instead of wasting time understanding it, they lazily project their own religious norms on others, even people who aren't religious at all. Thus, we have popular myths of atheists worshiping everything from Satan to Darwin to themselves. And a frighting chunk of believers buy into their every misinformed word.
So what is atheism?
Atheism is merely a response to the question, "Do you believe in the existence of a god?". Theists say yes, while atheists say no.
Atheists are simply people who disagree with theistic claims - claims of a Creator God who made the universe. That's it. There's not necessarily any other point of agreement.
Atheism isn't a philosophy or worldview or even a religion for the same reason that not believing in leprechauns represents a worldview. Atheism isn't even a belief, it's a rejection of a belief and the only real difference between it and a-leprechaunism or a-unicornism or a-fairyism is that very few people seriously believe in such things anymore.
That said, atheists do not "believe in nothing". Atheists can indeed construct valid and positive worldviews, but these views are not atheism, they are beliefs in addition to atheism.